A projection-type video display device, including a screen and a projection device which projects a video image on the screen, has been widely used. In a typical projection-type video display device, a two-dimensional image as a basic image is generated by using a spatial light modulator such as a liquid crystal micro display or a DMD (digital micromirror device), and the two-dimensional image is magnified and projected on a screen by using a projection optical system, so that a video is displayed on the screen.
As the projection device, various types including a commercialized product called an “optical type projector” have been proposed. In a general optical type projector, the spatial light modulator such as a liquid crystal display is illuminated by using an illumination device including a white light source such as a high pressure mercury lamp, and an obtained modulation image is magnified and projected on the screen by using lenses. For example, Patent Literature 1 listed below discloses a technique where a white light beam generated by a supper-high pressure mercury lamp is divided into three primary color components R, G, and B by a dichiroic mirror, the light beams are guided to spatial light modulators corresponding to the primary colors, generated modulation images corresponding to the primary colors are combined by a cross dichiroic prism to be projected on the screen.
As an application of the projection device and the projection-type video display device, a projection device which projects video image corresponding to each screen on a plurality of the screens is known. In addition, a technique where signals representing different images can be output from one information processing terminal is also known, and video is displayed on a plurality of the screens by using the technique and the projection device.
A plurality of the projection devices are needed in order to project the video image corresponding to each screen on a plurality of the screens. At this time, although a plurality of the spatial light modulators and a plurality of the projection optical systems are needed, it has been considered that the projection device can be miniaturized by commonly using the illumination device illuminating the spatial light modulators.
However, the illumination device of the related art uses a high pressure mercury lamp, an LED, or the like as a light source to spread a light beam of the light source and transmit the light beam. In the illumination device of the related art, since the light beam is spread, there is a loss in light intensity, and there is a problem in that an optical system becomes large.
Further, high intensity discharge lamp such as a high pressure mercury lamp has a relatively short lifecycle, and in the case where the lamp is used for an optical type projector or the like, the lamp needs to be frequently replaced. In addition, since a relatively large optical system such as a dichiroic mirror is needed in order to extract the light beams of the primary color components, there is a problem in that the size of the whole apparatus is increased as described above.
In order to cope with this problem, a type using a coherent light source such as a laser is also proposed. For example, a semiconductor laser which is widely used in industries has a very long lifecycle in comparison with the high intensity discharge lamp such as a high pressure mercury lamp. In addition, since the laser source is a light source capable of generating light having a single wavelength, a spectroscopic apparatus such as a dichiroic mirror is unnecessary, so that there is an advantage in that the whole apparatus can be miniaturized. Further, since the diameter of the light beam of the laser can be easily controlled, a loss of light may not easily occur.
On the other hand, in the type using the coherent light source such as a laser source, there is another problem in that speckles occur. The speckle is a punctate pattern which occurs when the coherent light beam such as a laser beam is irradiated on a scattering plane. If the speckle occurs on the screen, it is observed as punctate luminance irregularity (brightness irregularity), so that it becomes a factor of exerting physiologically bad influence on the observer. The reason why the speckles occur in the case of using the coherent light beam is that the coherent light beams reflected from portions of the scattering reflecting plane such as a screen have very high coherency, and the speckles are generated through interference therebetween. For example, in Non Patent Literature 1 listed below, theoretical review of the occurrence of speckles is made in detail.
In the type of using the coherent light source, since there is an intrinsic problem in that the speckles occur, techniques for suppressing the occurrence of speckles have been proposed. For example, Patent Literature 2 listed below discloses a technique where a scattering plate is irradiated with a laser beam, an obtained scattered light beam is guided to a spatial light modulator, and the scattering plate is driven to rotate by a motor, so that speckles are reduced.